The present invention generally relates to clock signal generation and particularly relates to synthesizing desired clock frequencies.
Frequency synthesis can be defined broadly as the act of generating a desired frequency from one or more reference frequencies. For example, frequency synthesis may be used to create one or more output frequencies using a single reference frequency. Other forms of frequency synthesis derived desired output frequencies based on the interaction of two or more reference signals, each at a different frequency.
As might be imagined, frequency synthesis in its various forms finds widespread use across a variety of system and device types. One exemplary use of frequency synthesis is the creation of one or more system clock signals from a single reference frequency signal. Synthesizing the system clock signals from the same reference frequency ensures that all clock timing relates back to the same base frequency. Such synchronization can be important in sampled data systems wherein the process of converting analog signals into digitized samples must be carried out in synchronization with a microprocessor clock or other system clock.
The use of multiple frequencies synchronized to the same baseline reference frequency also is important in many types of communication systems, including wired (electrical and optical), and wireless communication systems. For example, mobile terminals typically use multiple frequencies in their operations, including different frequencies for receive/transmit signal mixing, filtering, down-conversion, signal digitization, and baseband digital processing. Because such operations typically must be performed in synchronization with a baseline system timing, such mobile terminals typically have a reference clock running at a baseline frequency, and typically employ one or more frequency synthesizers to generate one or more additional clock signals having desired frequency relationships to the reference clock signal.
Of course, such aspects and applications of frequency synthesis are well known to those skilled in the art, and certain forms of frequency synthesis can be implemented using relatively straightforward circuits, such as the use of simple multipliers and/or dividers to achieve synthesized frequencies that are integer multiples or factors of the reference frequency, e.g., 2×, 3×, ½×, ¼×. However, synthesis becomes more complicated when the desired frequency relationship between the reference and output signals has a fractional component.